1. Field of the Invention
This invention relates to an ink jet recording apparatus that prints patterns on a recording medium by jetting ink droplets out of nozzle openings.
2. Background
An ink jet recording apparatus employs a recording head that is designed to jet an ink droplet out of a nozzle opening by applying pressure to ink within a pressure producing chamber using a piezoelectric vibrator or a heating element. As a result of the use of the thus constructed recording head, the ink jet recording apparatus requires that some measures be taken to prevent impairment in printing quality attributable to the drying of ink and deposition of dust in the vicinity of the nozzle openings.
Although such ink jet recording head comes in a variety of structures as shown in FIGS. 7a to 7c, the ink jet recording head basically includes: pressure producing chambers 33, 34, 35 that receive pressure from pressure producing elements 30, 31, 32; and nozzle plates 40, 41, 42 having nozzle openings 37, 38, 39 formed therein, the nozzle openings 37, 38, 39 communicating with the pressure producing chambers 33, 34, 35 either directly or through a passage 36.
Furthermore, a recording head shown in FIG. 7b will now be described as an example. In the recording head, a meniscus 43 of ink formed adjacent to the nozzle opening 38 of the nozzle plate 41 is in contact with the atmosphere through an opening entrance 38a as shown in FIG. 8a. Therefore, if the nozzle opening 28 is left unused with no ink droplet jetted therefrom, an ink solvent present in a region adjacent to the nozzle opening volatilizes from the nozzle opening to thereby increase ink concentration adjacent to the nozzle opening as shown in FIG. 8b. Then, the highly concentrated region (the densely dotted region in FIG. 8b) is spread over the pressure producing chamber 34 as time elapses as shown in FIG. 8c, and the viscosity of the entire portion of the ink within the pressure producing chamber 34 comes to be thickened to such a degree as to make it impossible to jet ink droplets in the end.
When the ink concentration adjacent to the nozzle opening becomes so high in this way, the viscosity of ink increases. However, since a force derived from the pressure applied to the pressure producing chamber by the pressure producing means is constant, the quantity of ink of an ink droplet to be jetted out of the nozzle opening is decreased, which in turn brings about an extremely grave impairment of printing quality.
Performed as one of measures to overcome this problem is a flushing operation. That is, in a condition in which the recording head is released from the capping means such as during printing or during a period for waiting for an input of data, i.e., in a condition in which ink adjacent to the nozzle openings is not replenished and in which the ink solvent is easy to volatilize from the nozzle openings, the recording head is moved to the ink receiving member in the nonprinting region, and ink droplets are thereafter jetted out of the nozzle openings by applying a drive signal to the pressure producing means independently of printing data at a predetermined cycle, e.g., every 20 seconds, so that the viscosity-thickened ink adjacent to the nozzle openings is discharged.
After such flushing operation and a series of printing operations have been terminated, a flushing operation is performed immediately before the recording head is sealed with the capping device. That is, this flushing operation flushes a number of ink droplets greater than that to be jetted by the periodic flushing operation during a period for which the recording head is released from the capping device (hereinafter referred to as "uncapped period" whenever applicable), so that the recording head is sealed with the capping device after the viscosity-thickened ink present adjacent to the nozzle openings have been discharged reliably.
By the way, ink used for a recording head that is particularly dedicated to printing with a quality equivalent to photography by preventing the penetration of ink into a recording medium, has the ink solvent thereof evaporated at an extremely high speed and also has high film forming properties. In addition, if an ink having a viscosity of about 4 mpa.multidot.s, which is greater than that of a conventional ink, is to be used, the evaporation of the ink solvent at the nozzle openings even to a scanty degree brings about inconvenience in jetting ink droplets out of the nozzle openings.
For overcoming such problem, a technique shown in FIG. 9 is taken. That is, if the uncapped time is short, the number of ink droplets is increased in proportion to the total time for which the recording head has been released from the capping device, and when the total time exceeds a predetermined time, a flushing operation before capping is performed by jetting the number of ink droplets fixed to a maximum number.
However, in the ink having high film forming properties and having a viscosity of about 4 mpa.multidot.s that is higher than the conventional ink, the viscosity-increased ink region tends to spread deep into the recording head as shown in FIG. 8c. Therefore, if the uncapped period during a printing operation including a single round of printing is made longer, the aforementioned technique in which the ceiling is put on the number of ink droplets to be flushed during flushing is no longer viable to discharge the viscosity-thickened ink reliably.
Further, in order to discharge such viscosity-thickened ink that has been spread deep into the recording head reliably, the number of ink droplets to be flushed must be increased to an extremely large value. In such a case, the problem that the ink is wasted and that the capping means becomes large-sized if the capping means is designed to serve also as a waste ink tank and further as an ink receiving member.
The invention has been made in view of the aforementioned problems. The object of the invention is, therefore, to provide an ink jet recording apparatus that can discharge viscosity-thickened ink within the recording head reliably by suppressing the quantity of ink to be consumed for flushing operation before capping.